Leading edge blade erosion in steam turbines is attributable to moisture droplets in the steam flow that impinge upon the blade leading edge. Various measures have been taken to reduce such blade erosion. For example, water catchers and drainage devices have been incorporated in turbine walls; baffles and drainage passages have been incorporated in stationary blades; and grooves, stelliting, and surface-hardening have been used on rotating blades. While various methods such as these have been successful in somewhat alleviating erosion, such erosion continues to be a problem in steam turbines.
A study conducted several years ago and reported in ASME Paper No. 63-WA-238 entitled "Tangential Blade Velocity and Secondary Flow Field Effect on Steam-Turbine, Exhaust-Blade Erosion", published November, 1963, describes the secondary flow field effect and how it contributes to moisture transport in the steam flow. Secondary flow in a cylinder blade row (stationary blades) is generated by the static pressure gradient along the end wall which confines the main steam flow field within the boundaries of the suction and pressure surfaces of adjacent blades. The static pressure gradient imposed upon the end wall boundary layer fluid causes the boundary layer to flow along the end wall from the pressure side of one blade to the suction side of an adjacent blade. The secondary flow pattern on the blade suction side has a radially inward component tending to spread the accumulated moisture along the trailing edge of the blade. The radially inward depth of the secondary flow varies with end wall shape. For a cylindrical end wall, the depth is between about 10% and 15% of blade length while for an S-shaped end wall, the depth may be as high as 25% of blade length. The blade erosion pattern on rotating blades immediately downstream of the cylinder blades correlates with the depth of secondary flow.
Cylinder blade pitch also affects secondary flow and the depth of erosion on rotating blades. Increasing pitch produces a concomitant increase in secondary flow. When cylinder blades are pitched properly, secondary flow is primarily axial in orientation and erosion depth on rotating blades is reduced. Overpitched cylinder blades result in a secondary flow with a significant radially inward component resulting in increased depth of erosion. However, even with properly pitched blade and axial secondary flow, moisture will accumulate at a significant radial distance from the rotating blade tip because of the radially outward divergence of the end wall of the stationary blades.